BRIEF BIOGRAPHY OF JOHANNES KEPLER

Johannes Kepler was both a remarkable and a curious character, and his life was full of contrasts. He had an abominable family background, had few practical skills and yet he occupies a unique position in the history of science. He was frequently ill and yet he belongs to that highly select group of gifted individuals whose intellect shines from a truly fragile body; he overcame all the difficulties in his path with incredible resiliency. He was both in advance of, and behind, his contemporaries. Some of his ideas were truly revolutionary, but in contrast, others were rooted back to the mediaeval past. He was truly a Janus.

Kepler was the first 'physical' thinker in astronomy; in all phenomena he looked for the causes. On observing some phenomenon, the traditional question would be ... "What does it mean?" But Kepler would ask ... "What is it and what's the cause?" He asked the questions that nobody else raised and looked for answers where nobody else saw problems! What he did, then, was to combine 'celestial' and 'terrestrial' physics; clearly, this was a different method of thinking. He was not concerned so much with understanding God's purpose in creating the universe but finding: (a) the laws that were in the mind of the Creator; as far as Kepler was concerned these laws could only be geometrical, and (b) the physical means used by the Creator to keep the system together and in motion. According to Kepler this was by no means crazy idea; Man's soul was created with the image and likeness of God so why shouldn't an Earthly mathematician be capable of penetrating, if only partially, the mind of the Divine Mathematician? For Kepler, understanding nature is nothing but thinking in accordance with the thoughts of God, who always pursues geometry.

Johannes Kepler was born on December 27, 1571 at Weil der Stadt in Württemberg, South West Germany; he was baptized Johannes after the saint of the day. The Kepler family had been located in Weil der Stadt for about fifty years. His paternal grandfather, Sebald Kepler, was a craftsman and had served as a local burgomaster and a much respected individual. His maternal grandfather, Melchior Guldenmann, was an innkeeper and mayor of the nearby village of Eltingen. But the family's reputation declined. Kepler's mother, Katherine, appears to have been a strange woman, an interfering busybody with a malicious tongue; in notes compiled later he described her as ... "small, thin and dark-complexioned, garrulous, quarrelsome and generally unpleasant". She gathered herbs and prepared salves and believed in magical forces, beliefs and acts which were to cause her much trouble in later life when she was accused of being a sorceress and witch. His father, Heinrich was shiftless, irresponsible and bad-tempered and spent a good deal of time away from home volunteering as a soldier; Kepler described his father later as ... "an immoral, rough and quarrelsome soldier". Johannes was the eldest of the Kepler's seven children; it seems that only three other Kepler children survived to adulthood [1]. In 1574 his father left to fight in the Netherlands and in the next year his mother joined him. Johannes was essentially abandoned to be looked after by his grandparents, who cared little for him and treated him roughly. In the two years he spent with them he was very ill with smallpox and almost died. His parents returned in 1576 and settled in nearby Leonberg. Johannes joined them but his father returned to his wayward habits and finally left for good in 1588. He remembered two events from his boyhood that stimulated his interest in astronomy:

They had such an effect on him that he was able to recall the details in later life.

Home life could have hardly been happy and peaceful for Johannes; in a sense, it set the unsettled life he was to experience later. He was a small, sickly child and illness left him with poor eyesight. Fortunately, Württemberg had good schools, both 'German' and 'Latin' schools. It seems that Johannes first attended the local German school but the teachers there were very content to send their more talented pupils to the Latin school. He entered the first class at age seven years but altogether he took about five years to complete three classes, not because he found the work difficult - on the contrary, he demonstrated exceptional academic abilities and did well at school. His parents frequently removed him from school, once when they moved and on other occasions when they forced him into work.

After completing his studies at the Latin school it was obvious he was too weak for regular labor. In view of his obvious ability and piety, his teachers suggested a future in the Lutheran clergy and Johannes happily agreed. On October 16, 1584, after passing the entrance examinations, he entered the convent-school Adelberg. He was an unusual student, not so much because of his abilities and achievements, but he was so highly introspective that he tortured himself with self-observation. Although he formed occasional friendships he often disagreed and quarreled with the other students. He thought he was created to think about the 'difficult things' that others turned away from [2], and his religious convictions were so pronounced that they were often extreme and exaggerated.

From there, on September 17, 1589 he entered the University of Tübingen - an ideal place for a theologian - on a scholarship. Life was plain and strictly regulated; the founder of the seminary had ordained that the scholars should be 'children of poor, pious people, with an industrious, Christian and God-fearing character.' Here, Kepler was in his element; he earned the reputation of being diligent and sedate and he did well academically although he suffered bouts of illness. On August 10, 1591 he passed the master's examination - he was second in a class of fourteen -and his scholarship was renewed. Although Kepler had yet to develop a deep interest in astronomy, he came under the influence of Michael Mästlin, a professor of mathematics and astronomy at the university who was to become a life-long friend [3]. Mästlin soon realized there was something special about Kepler, who showed a real interest in mathematics. He introduced Kepler to Copernicus' ideas and Kepler became convinced of the essential correctness of the theory, placing the Sun at, or near, the center of the universe. (Mästlin may well have held the same view but he could not admit it; officially the Ptolemaic theory was accepted and to have admitted a belief in Copernicanism would have risked his position and the peace and order of the College [4].) Kepler wrote later:

Kepler was happy to work on astronomical questions and he also maintained his interest in astrology, becoming prominent among students as a master in casting horoscopes. However, he had not gone to Tübingen to become and astronomer, mathematician or philosopher but to prepare himself for a theological position. How was it that the free-thinking, intelligent, perceptive Kepler could adhere to the strict doctrines and rules of the Church? The objections he had to the conventional thinking certainly worried him, so that he was forced to sweep away completely his worries when he attended Holy Communion. However, he was able to overcome some of the difficulties by reading various texts and commentaries but he says that ... little by little he grew to hate the controversy.

In 1594, a position at the Protestant seminary at Graz (in Austria) for a teacher in mathematics and astronomy became vacant, combined with the post of mathematician to the Province of Styria. The provincial authorities sought a promising individual from Tübingen, and Kepler was selected by the Senate. He was astonished by the decision as he had believed that he would become a priest. He felt ill-prepared to teach mathematics although he understood fully the astronomy and geometry that the position demanded. Despite these misgivings he left the University on March 13 and arrived at Graz on April 11 [5]. (Later he was to believe that this was a blessing; he felt that God had called him and it is God who guides Man to partake in the divine providence.) It was his first proper position but it was not a great success for a number of reasons; the rather low level of teaching bored him, he had few students - mathematics was not popular with the sons of nobles - and he was a poor teacher - much too demanding. Furthermore, in Württemberg the population had been unreservedly committed to Luther and so it was a stronghold of Protestantism, whereas in Graz, there was some religious tension; the rulers held rigidly to Catholicism and considered it their duty to suppress the new doctrine and lead their flock back to the old beliefs. His official duties as mathematician to the Province involved him with astrology, casting horoscopes and producing calendars - the first was for 1595 - giving assorted prognostications from the dates of eclipses about war, religious and political events, when to 'bleed', to the best times for collecting herbs, etc. The first calendar proved to be remarkably accurate - predicting among other things, cold weather and invasions of the Turks - so that he was given an additional honorarium. Altogether he produced five more calendars. However, he sought much more intellectual stimulation and he was able to find time to continue his own private studies.

At Graz he began his quest for the 'harmony of the universe', seeking the underlying rhythm and design of the Solar System. At the time, the prevailing thought was that orbits - no matter whether it was a Sun-centered or an Earth-centered system - were circular but were not centered on either the Sun or the Earth, and this would allow for variations in the distances of the planets. It was then suggested that each planet must have a 'spherical shell' or region of space in which it must always be found. So that no space could be wasted, it was thought that these spherical shells must touch each other; thus, the outer boundary of Jupiter's sphere, i.e., the greatest distance of Jupiter from the Earth, much touch the inner boundary of the sphere of Saturn, etc. Although very neat, Copernicus had shown that this was not true and Kepler thought particularly deeply about this problem. In the early part of 1595 he wondered, for example, why there were only five planets (excluding the Earth) and whether there was any connection with Euclid's five regular solids of geometry, i.e., the (4-sided) tetrahedron, the (6-sided) cube, the (8-sided) octahedron, the (12-sided) dodecahedron and the (20-sided) icosahedron. He tried various geometrical constructions that would produce the distances between the Sun and the planets. He worked with little success during the summer but, apparently, on July 19, 1595 - he recorded the great day - during a teaching session he found the solution. He said:

Kepler spent the winter of 1595-6 working on his theory often seeking help and advice from Mästlin. In early 1596 he took leave of absence of absence and traveled to back to his homeland. Both grandfathers were old and sick and had wanted to see their grandson. He also visited Mästlin in Tübingen to discuss the finer points of his theories. Mästlin had said that the work Kepler had done was highly ingenious, thoroughly worthy of publication and entirely new although he was critical of the unclear and at times confused presentation that assumed the reader was familiar with all of Copernicus' theory. Kepler made changes and expanded the text in various places. The senate of the University of Tübingen had approved the book and printing began under the watchful eye of Mästlin. Kepler returned to Graz in August 1596; initially, he had asked for and been granted only two months leave but in all he had been away for almost seven months. His superiors forgave him because they were assured that the work he had been carrying out and the book that he was about to publish was of great significance.

In the Spring of 1597 Kepler received the initial copies of his book, which he had the extremely grand title Prodromus Dissertationum Cosmographicarum continens Mysterium Cosmographicum de admirabili Proportione Orbium Coelestium deque Causis Coelestium numeri, magnitudinis, motuumque periodicorum genuinis et propriis, demonstratum per quinque regularia corpora Geometrica, now referred to with the shortened title Mysterium Cosmographicum (Mystery of the Universe). In it, he postulated an extremely complicated way of fitting within each other spheres of finite thickness, on which the planets moved, and the five regular solids. He wrote:

He realized that the agreement between the distances in his construction and the known astronomical data was only very approximate. However, he felt that the agreement was too significant to be ignored. However, he was very excited and surprised that he, a sinful person, should be allowed to lift the veil that hides the divine design.

In addition to the geometrical ideas in the Mysterium Cosmographicum Kepler also commented on the movement of the planets around the Sun. Contemporary thinking was that motion would cease unless a constant force was applied and so Kepler supposed that the Sun exerted a force that caused the planets to move. Furthermore, he reasoned that the effects of the force decreased with distance, which explains why the most remote planet, Saturn, is the slowest moving member of the system. (It is interesting to note how this idea some 20-25 years later developed into his third law.)

In reality, the book has little scientific value but it did have a major effect on Kepler's future career. Two copies were brought to Italy by Kepler's friend Paul Hamberger and both were left with Galileo, at Padua, on the eve of Hamberger's return to Germany. He carried a note of thanks to Kepler from Galileo, dated August 4, 1597 [6]. Kepler wrote back urging Galileo to support publicly the Copernican theory, asking him to carry out some measurements and to seek his opinion of the book. Galileo did not reply, possibly because he was not yet fully interested in astronomy.

Of more importance, for Kepler, was the contact he made with Tycho Brähe. Kepler sent Brähe a copy of the book and a covering letter but about the time Mysterium Cosmographicum was published Brähe's period at Uraniborg was coming to an end. Eventually, Kepler's book reached Brähe while the latter was in Germany. Although he could certainly not agree with Kepler's constructions, clearly Brähe felt that the young man was a highly gifted mathematician. He wrote a long letter to Kepler that was a carefully balanced criticism of the book and offering him the prospect of a position [7]. Therefore, despite the uncertain validity of his ideas, Kepler had succeeded in making contact with and impressing the two most important astronomers of the time.

On February 9, 1597 Kepler made a solemn promise of marriage to Barbara (nee Müller) who was pretty and plump, and despite her youth - she was in her twenty-third year - had been widowed twice, with one daughter, Regina, from her first marriage. On April 27, 1597 they were married. On February 2, 1598 a son was born, christened Heinrich. However, the boy died after sixty days and a daughter, born in June 1599, survived only thirty-five days. At this time, in addition to his family troubles, Kepler was also being drawn into the conflict between the Protestants and Catholics in the region. Shortly before Kepler's marriage, in December 1596, the young Archduke Ferdinand had assumed the role of ruler. In early 1598 Ferdinand traveled to Italy and met the Pope. While in Italy he vowed to lead his country back into Catholicism and rumors spread throughout Graz. Kepler wrote:

The Counter Reformation had begun [8]. On September 13, 1598 an order was published to dismiss preachers and abolish the entire collegiate chapter and school ministries in Graz and other cities within fourteen days. A request for the repeal of the order was refused and on September 23 the preachers and collegiate teachers were ordered to leave the city in eight days. After another fruitless appeal a more severe demand was made on September 28 that collegiate preachers, rectors and school employees depart that very day from Graz. Kepler left, but an exception was made in his case and he was allowed back to Graz at the end of October. However, the die was cast; life in Graz was becoming more and more oppressive - for example, he was denied the services of his Christian beliefs [9] and although he received a salary it was very small - since the school where he had worked was closed - and so he could not expect any increases. It was not long before all Protestant clergyman still in the area were expelled and anyone who sang hymns or read Luther's bible made himself liable to banishment; soon it became apparent that all Lutherans would be expelled and they would lose their right to trade or sell possessions in the city. Against this background Kepler continued 'scholarly' investigations relating to the chronology of the Old Testament (with Mästlin); the arrangement of stars at the time of their Creation, at the time of the civil war between Julius Caesar and Pompey and at the birth of Augustus, etc. Although he gained knowledge from these studies, clearly, he had to seek a more secure future. He was not able to secure a position at Tübingen and so he began to think seriously of taking up Tycho Brähe's invitation [10].

During this period Brähe had been called to Prague as the court mathematician by Emperor Rudolph, arriving there in June 1599. (Shortly afterwards, in August, he moved to the Castle at Benatky, some twenty-two mile north-east of Prague, as he was unable to work in Prague itself.) In a friendly letter to Kepler in December, Brähe repeated his invitation and in January 1600 Kepler traveled to Prague. On February 4 the two great men met face to face for the first time. Their discussions were not harmonious - perhaps, to be expected since their backgrounds, life-style and status were so different as were their ages (Kepler was 28 and Brähe was 53) - but eventually they came to some agreement, albeit uneasy, as Brähe was almost desperately seeking a reliable and knowledgeable collaborator and Kepler had many novel ideas but needed copies of Brähe's accurate data (although the latter would not permit that). Kepler had a number of concerns; he realized that he needed to work with Brähe for a much longer period than he first thought, he was uncertain about where to live - he preferred Prague but Brähe wanted him to live either at Benatky or back in Styria - and he felt he would be better served if he was employed by the Emperor rather than by Brähe himself. Furthermore, how was he going to arrange getting the consent of his employers for a long absence? During April a real difference of opinion occurred between the two, which was only patched up after apologies and a change in attitude, particularly on the part of Kepler. He continued to work for a few more weeks with Brähe but after making arrangements for longer collaboration and gaining other promises [11], he left Prague on June 1 and returned to Graz.

In reality, it was the circumstances in Graz that forced Kepler's hand. On July 27, 1600 Archduke Ferdinand published a decree summoning most of the population of Graz to appear in front of the city magistrate on July 31 for examination concerning their faith. Anyone who was not Catholic, or did not pledge himself to become Catholic and go to confession and communion, was to be banished after paying 10% of their assets. The process took three days; Kepler appeared before the magistrates on August 2. He refused to become a Catholic and so he was banished. Accordingly, his employment was terminated on August 12, but he was given a written reference, praising him highly for his work as a professor. He had to act quickly, so, he sold his property, wrote to Brähe - who encouraged Kepler to join him [12] - and on September 30, 1600 he left Graz with his wife and stepdaughter and two wagons that carried his possessions, bound for Prague.

Kepler arrived in Prague depressed and in poor physical shape on October 19. He had again been unable to get a position at Tübingen, even in the hope of following a medical career, and so he threw himself on Brähe's and the Emperor's mercy, seeking a position that would provide financial support for him and his family. He was in very poor health but in the Spring of 1601 he returned to Graz on the death of his father-in-law Jöbst Müller and he wanted to look after his wife's interests and convert her fortune into cash. He remained in Graz for a few months, returning to Prague in September. Shortly afterwards Brähe introduced him to the Emperor, who commissioned him to collaborate with Brähe in compiling a new set of tables, which the latter suggested should be called the Rudolphine Tables. However, on October 24, 1601, after a short illness, Brähe died of a bladder ailment. Before his death, Brähe begged Kepler - to whom he wanted to leave his scientific data - to demonstrate the validity of the Tychonic system. The Emperor agreed to the transfer of Brähe's incomplete works and instruments to Kepler and appointed him Brähe's successor as Imperial mathematician, albeit at a much smaller salary than Brähe. Ever-believing in the intervention of the divine, Kepler said:

Despite all the problems he faced, it was now that the 'real theoretician' in Kepler came to the fore, when he returned to the problems of the orbit of Mars [13]. Eventually, by comparing careful calculations with Brähe's data he deduced that both the Copernican and the Tychonic systems - based on circles - were incorrect. In 1602 he determined that:

which is now known as Kepler's second Law. Around Easter 1605, after much calculation - and a severe bout of depression in December 1604 with thoughts of death - he finally solved the riddle of the shape of the orbit of Mars [14].

By calculating the deviations of the true orbit from a circle he determined that, and assuming that what held for Mars must be true for the other planets:

which is now known as Kepler's first Law. Several of his closest colleagues, Michael Mästlin, David Fabricius [15] and Longomontanus, were skeptical; later, even Galileo took no notice of Kepler's results. Nevertheless, he collected his researches and wrote a manuscript Astronomia Nova (New Astronomy), which he completed in 1605. The publication was delayed because of problems over the shortage of funds in the Imperial treasury and the agreements he had made with Brähe's heirs, particularly Brähe's son-in-law, Tengnagel [16]. Printing started in 1608 at Ernst Vögelin's in Heidelberg and was completed in the summer of 1609. Kepler was denied the public sale of the book by the Emperor, who demanded that ... "he give no one a copy of it without our previous knowledge and consent." He reserved these rights because it was written by Kepler in his official capacity as Imperial mathematician and printed with Imperial money (and probably because he thought it was an important book!). In addition to the two laws, the book also a number of other ideas, including the first of the modern ideas on gravitation (and tides):

During the early 1600's Kepler also made studies of optics. He investigated a wide range of subjects including the nature of light, the path of rays in the eye, the explanation of the operation of eyeglasses for correcting near- and far-sightedness, an explanation of the luminous appearances around the Sun at a total solar eclipse and atmospheric refraction (although the did not succeed in deriving the law of refraction; that was done later - in 1620 - by Willebrord Snell). It contains only a few 'new' researches; it is a good digest of earlier studies by such men as Ptolemy and Alhazen (ca. 950-1039). He completed the main body of work in 1603, however, he was interrupted by his work on Mars and so the manuscript of his Astronomia Pars Optica was not presented to the Emperor until early 1604.

At dawn on October 11, 1604 an Imperial official responsible for weather observations saw a brilliant new star - a supernova - in the constellation of Ophiuchus (the Serpent-Bearer) and close to a conjunction of Saturn, Jupiter and Mars. Bad weather intervened so that Kepler was not able to see the phenomenon himself until October 17. He arranged for continuous observation and some two years later he published his findings in a book, Stella Nova, dedicated to the Emperor. It contains not only a good deal of astronomically important details but also a considerable amount of fanciful astrology.

In 1607 he published his Phaenomenon singulare in which he mistakenly refers to a transit of the planet Mercury across the Sun's disk. According to his calculations on May 29, 1607 Mercury would pass directly between the Earth and the Sun, thus, appearing as a dark spot against the bright Sun. He imaged the Sun through a narrow hole onto a paper screen and he saw:

on the bright image of the Sun. He repeated the observation:

Kepler was convinced he had seen the transit but he had not; what he had seen was a large Sun spot. It did not take him long to realize that he had indeed made an error. (He predicted another transit of Mercury in 1631, which was observed by the French astronomer Pierre Gassendi.)

His personal level during this period could be hardly described as smooth and conventional. He had considerable financial difficulties mainly through irregular payments of salary (although he was the only court employee to suffer this way). For example, in 1607 he wrote:

As a result he was unable to employ a much-needed permanent assistant although from time-to-time he did have part-time helpers. His domestic life was simple, but his marriage to Barbara was not very happy. Although she made a good impression in public, Barbara was not so charming at home. According to Kepler she had:

Kepler excused her weaknesses because she was sick. She had three children with Kepler, Susanna (born July 9, 1602), Frederich (born December 3, 1604) and Ludwig (born December 21, 1607). In the Spring 1608 his step-daughter, Regina, was married; she died, aged 27, on October 4, 1617.

Throughout his life Kepler was very much influenced by the religious state of affairs where he lived; we have seen that he was forced to leave Graz because of his faith. In Prague, he was essentially left alone, despite the religious turmoil that city had experienced [17]. The main reason was Rudolph's deep interest in the arts and sciences; he simply did not concern himself with the religious beliefs of the scholars at court. Indeed, within Prague questions of art, politics, religion and science were keenly discussed and differences of opinion were actually favored. In addition, Kepler was held in very high esteem.

The end of the first decade of the 17th century was a turbulent time for Kepler, both scientifically speaking and in determining his future. In March 1610 he heard that in Padua Galileo, using a two-lens perspicillum or telescope, to give it its modern name, with a thirty-fold magnification, had discovered four new planets. On April 8, 1610 he received a copy of the Siderius Nuncius from Galileo, describing his recent discoveries, and was asked his opinion. Kepler was overwhelmed by these discoveries and wrote a letter to Galileo, dated April 19, that later was published with the title Dissertatio cum Nuncio Siderio (Conversation with the Starry Messenger). He says he takes up his pen so that Galileo:

The contrast between Kepler and Galileo is well illustrated by their two publications. In his Siderius Nuncius Galileo maintains a precise investigation and representation of the facts. Kepler, on the other hand, in his Dissertatio looks ahead and speculates ... why should Jupiter be the only planet with satellites? ... does symmetry demand that Mars has two? ... what purpose do these satellites serve? Above all, Kepler wanted a telescope, but the ones available in Prague were useless. Apparently Galileo was approached but nothing came of it. Eventually, Kepler obtained one of Galileo's telescopes, albeit indirectly, and he observed Jupiter from August 30 to September 9. He published his results in a booklet Narratio de Jovis Satellitibus, which was reprinted in Florence and provided a strong confirmation of Galileo's discovery.

During August and September 1610 Kepler wrote a book, Dioptrice, describing all of the laws governing the passage of light through lenses and lens systems. It covered a wide range of subject matter and makes Kepler arguably the 'father of modern optics'. For instance, he discusses real and inverted images, magnification and shows:

This is the basis of the so-called astronomical telescope, now often called a Keplerian telescope. Later he discusses the Galilean-type telescope, i.e., with a convex (or converging) objective lens and a concave (or diverging) eyepiece lens. He finished the manuscript in September 1610 but the printing was much delayed.

In the meantime Galileo was continuing his observations using the telescope. He did not publish his results in book form but in August and December 1610 Kepler received information about discoveries concerning the possibility of moons around Saturn and the phases Venus, respectively. The message about Saturn read:

while that about Venus was in the form of an anagram:

Kepler, who as we had seen above thought that Mars might have two moons, managed to unscramble the letters of the first message and obtained (incorrectly, as it turned out!):

However, in November Galileo sent the solution to the Emperor, who had also become interested. The true message read:

On January 1, 1611 Galileo sent the solution to the Venus anagram to the Emperor:

The postponement of the publication of Dioptrice allowed Kepler to add a preface announcing Galileo's discoveries. His unbounded enthusiasm is obvious when writes:

Apparently, after the publication of Dioptrice Kepler took no further interest in optics.

The year 1611 proved to be a very difficult year for Kepler. Political developments in Prague led to disorder and tension. At the end of 1610 Barbara was very sick with fever and epilepsy. Then, in January 1611 his three children caught smallpox. Although Susanna and Ludwig survived, Frederick died on February 19, aged six years. All this time Rudolph was losing his grip and he was finally forced to abdicate on May 23, 1611 in favor of Archduke Matthias. Kepler was active in these events; he remained loyal to the Emperor although his religious feelings meant he was closer to the other side. He was called upon on a number occasions by followers of both sides to give his expert astrological opinion of events and the future. Wisely, Kepler remained diplomatic. Clearly, it was time for him to seek a new home. In March he turned again to the duke of Württemberg (his home). Outwardly, he looked highly suited to an honorary position at Tübingen and since Michael Mästlin was now old, he could be considered as a replacement. However, on further scrutiny, his religious views were found to be inconsistent with the statutes of the University that required teachers to be followers of the Augsburg Confession, which Kepler had earlier denied. Indeed, he was thought to be a 'sly' Calvinist who might inspire and poison the youth with Calvinism. So, in April 1611 he was rejected and he realized that he could never return permanently to his homeland.

There had been the slim prospect of him replacing Galileo at Padua after the latter's departure for Rome. Although Galileo had recommended Kepler for the position, it came to nothing. Earlier, in December 1610, he had been invited to Linz, the capital of Upper Austria, and so at the end of May, immediately after the abdication of Rudolph, he went there to offer himself for the position of district mathematician. The arrangements were completed in June; he hoped that Linz would please his wife more than Prague had done since, there, she would be much closer to Graz. Sadly, on his return to Prague he found his wife seriously ill, apparently from a contagious disease - spotted typhus - carried by the invading Austrian troops. On July 3, she died. To compound his wretchedness, Barbara had not made a will and so in addition to suddenly having to care for his children he had the difficult task of dividing the estate between his remaining two children and his step-daughter, which resulted in great friction with his son-in-law. At the request of Rudolph, Kepler did not move to Linz straightway; he remained in Prague until April 1612, about three months after the deposed monarch's death on January 20.

Kepler was to spend about 14 years in Linz but, like most of his earlier life, it was full of 'highs' and 'lows'. His position, which had been created for him, was not entirely to his liking; he still had to subordinate himself to his superiors there and his duties also included teaching in the district school. However, Matthias (in Prague) had confirmed Kepler as court mathematician and provided a salary and so, in order to move and work in Linz, Kepler had sought Matthias' consent, which, of course, was granted (although Matthias did call upon him from time-to-time for counsel.) Kepler had hoped that in Linz he might complete the Rudolphine Tables.

The first of several problems Kepler was to face occurred within a few weeks of his arrival. As soon as he reached Linz he sought communion from his minister. He talked about his beliefs whereupon he was declared an "unhealthy sheep" with leanings towards Calvinism. Kepler was asked to sign a document that committed him to the 'Formula of Concord', he declined and so was refused communion, which troubled him greatly.

A second serious problem arose during 1615-16 when six women were condemned as witches in Leonberg, where his mother, Katherine, lived. She became involved with the tribunal also, having been accused by several members of the local community of casting spells, bewitching livestock, causing illness and death. As a result, a great deal of information was gathered against her. The accusations were led by Ursula Reinbold, an unbalanced woman with a highly questionable reputation who had been found guilty of public prostitution, and her family [19]. When accused in court Frau Kepler stood firm and refused a demand to return Reinbold to full health. Frau Kepler, with the support of her son Christoph and daughter Margarete, then submitted a complaint because of the insult. It was at this stage, on December 29, 1615, that Johannes Kepler was informed of the goings-on in Leonberg. On January 2, 1616 he sent a sharp letter to the counselor of Leonberg, outraged at the accusations against his mother. A trial was fixed for October 21, 1616 but another alleged serious incident involving Frau Kepler shortly before this date resulted in a further delay. In view of the increased seriousness of the accusations Frau Kepler was persuaded by her family to visit Johannes in Linz, where she arrived on December 13. Naturally she was accused of fleeing from her accusers but Kepler wrote to the duke giving excellent reasons for his mother's journey. She returned home in October 1617 and Johannes joined her a little later. (Interestingly, he took Dialogo della musica antica e moderna (Dialogue concerning ancient and modern music) by Vincenzo Galilei (Galileo's father) as reading material! He obtained permission to take his mother back to Linz with him but she wanted to remain in surroundings familiar to her since childhood. Kepler arrived back in Linz on December 22, 1617. At last, in May 1618, some three years after the original accusations, the formal examination of witnesses began. The proceedings lasted almost two years although there was a gap from the Fall 1618 to the Fall 1619. Some 30-40 witnesses were called. On July 24, 1620 Frau Kepler was ordered arrested, which was duly carried out on August 7. She continued to deny all the charges but the chief council demanded that she confess freely or be tortured on the rack. Kepler himself provided the defense, presenting a concluding statement on August 22, 1621. The trial was brought to a swift conclusion; the duke arranged for all the documents to be sent to the judicial faculty at the University of Tübingen where a 'dispassionate' opinion could be obtained. On September 10 the Tübingen college found that the evidence was not sufficient for the use of the rack nor could it exonerate Frau Kepler completely. They compromised by suggesting that she be shown the instruments of torture that would await her should she not confess. This procedure was carried out on September 28, 1621 but Frau Kepler stood firm. The duke then ordered her to be absolved from the complaints and released. She was set free on October 4, 1621. The following year, on April 13, Frau Kepler died in obscurity.

However, there were a number of more positive turns. In the autumn of 1611, a few months after his wife's death he contemplated marriage to a widow who had been introduced to him by his wife. But nothing came of the plan. Other possible matches arose; indeed, it seems that he considered a total of eleven candidates during the two years following his wife's death. He seemed to be tormented by the situation he finds himself in! He asks:

It is almost pitiful the helplessness, indecision and lack of self-assurance he exhibits in search of a wife! Eventually he 'selected' Susanna Reuttinger - aged twenty-four - an orphan from a neighboring town. The wedding took place on October 30, 1613, and since Kepler was the court mathematician he invited Archduke Matthias, his imperial master, to the reception. Kepler is full of praise for his wife although public opinion was not entirely sympathetic; even his step-daughter Regina cautioned him against the marriage. So, at last, Kepler could have his two surviving children - Susanna, aged eleven, and Ludwig, who was six - with him.

The marriage was very happy and during their stay in Linz Susanna produced six children; however, the first three died at a young age [20]. Kepler was a thoughtful father; he involved himself in his children's education and religious instruction, celebrating family prayers every Sunday.

Kepler continued his scholarly activities although there was no one he could discuss his astronomical interests. Nevertheless, he produced several books (although the trial of his mother and his deep sorrow over the death of his daughter, Katherine, interrupted his work and caused him to modify his plans).

Surely, now that these books were out of the way Kepler could concentrate fully on completing his great task - the Rudolphine Tables - which he had planned with Brähe almost twenty years previously. However, before they were finally published a number of difficulties arose - not the least of which was the Thirty Years War, which had begun in 1618 and devastated much of Germany and Austria - that intervened to disrupt his relatively peaceful existence and put his position in Linz in jeopardy. He said he was:

In addition he said:

In 1617 Kepler saw a copy of John Napier's famous work Mirifici Logarithmorum Canonis Descripto on logarithms, which had been published in 1614. He realized their importance immediately in reducing the time-consuming tasks of computation in astronomy and he thought that anyone using his Tables should have at their disposal such aids. Accordingly, in 1621-1622 Kepler composed a book that was eventually published in 1623, putting logarithm tables in the form suitable for astronomical tables. (In fact, some 200 years later, the French mathematician Laplace said that ... by shortening the labors, [logarithms] ... doubled the life of the astronomer.")

In spite of the long delay, the Brähe family was still jealous of Kepler even though the most awkward of Brähešs sons, Tengnagel, had died in 1622. The 'new' representative of the family Tychošs son, George, kept a watchful eye on progress to make sure that his fatheršs fame was maintained. Kepler's hands were still tied to the declarations he had made in 1604 and 1612 in which he pledged to submit the manuscript for inspection by the family, or a representative, before publication. The heirs wanted to pay him for his troubles but Kepler insisted that the Emperor had been his patron and that he, Kepler should take proper credit for the work. In addition, a problem arose over the place of printing; the printing presses in Linz were inadequate. His preference was for Ulm, which was well disposed towards him; however, there was still the problem over the costs of printing. So, in October 1624, he sought help from the imperial court in Vienna. Although an agreement was reached over the financial situation, the emperor insisted that the Tables be printed in Austria. So he was forced to buy printing presses - at his own expense - to improve those in Linz. However, before the work began in earnest, the Counter Reformation [8] once again, as had happened in Graz some years earlier, was to bring pressure on Protestants. Up to that time the Counter Reformation measures had not been strictly enforced but on October 10, 1625 an expulsion order on "preachers and un-Catholic schoolmasters" was renewed and they were forbidden to remain under threat "of heavy bodily and capital punishment." All heretical books had to be handed over and all inhabitants had to attend Catholic services; anyone who did not want to become Catholic could leave freely, but delayed departure would result in expulsion and a financial penalty. However, Kepler, as a court official, was exempted although he had to have his son Hildebert christened by a Catholic clergyman. Despite his immunity, he was devastated when almost all of his own library of books were put under seal by the Reformation commission. As a result, the printing of the Tables progressed very slowly.

Then the war intervened; in the spring of 1626 the peasants in Upper Austria, of which Linz was the capital, angered by the compulsory conversions imposed on them, revolted. On June 24 Linz itself was blockaded. The siege, which lasted until August 29, brought terrible suffering in Linz. On June 30 the peasants started a fire that destroyed a number of residences in the outskirts of the city including, sadly for Kepler, the printing presses. After the siege Kepler wrote to the Emperor seeking permission to continue the printing in Ulm; the Emperor could not deny him and so he packed up everything he owned - including his books, manuscripts and printing materials - and, accompanied by his family, he left Linz on November 20.

The printing in Ulm was not without problems. Kepler had to work closely with the printer, but found him to be an unpleasant, extravagant and impetuous person. In addition, the costs were much higher than he expected. However, Kepler found temporary employment assisting in the regulation and defining of weights and measures in the city. At last, in September 1627, the printing of 1000 copies of the first edition of the Tables was completed. Even then the Brähe family was not content; they were upset about certain phrases in the title and dedication, indeed, anything that was faintly critical of Tycho. Minor changes then required a second and third reprinting. Kepler considered the Tables as his principal astronomical work although he knew that there were still some unexplained deviations and disagreements with observations. Still, they remained the premier tables for more than a century. Although he must have felt a great deal of satisfaction in completing finally the Tables, late in 1627 he was faced with the question of where to go next. What he wanted to do was to travel and lecture about the Tables. However, that didn't materialize but he did have an offer from Landgrave Philip of Hesse (to whom he had dedicated his work on logarithms). After meeting up with his family in Regensberg, after a year's absence, he arrived in Prague on December 29, 1627. Much had changed in Prague and he was well received; so much so that he decided not to take the Landgrave's offer. Nevertheless, Kepler's view of the "Catholic" church was not consistent with that of the Emperor and so again, he could not remain in Prague. He moved to the Duchy of Sagan in Silesia at the invitation of Wallenstein, a powerful statesman and soldier, arriving on July 29, 1628. In fact, their paths had crossed in 1608 when Kepler was asked to cast Wallenstein's horoscope. Wallenstein had been given the Duchy by the Emperor and he was keen to raise its stature, so he was delighted to have the council of such a famous man as Kepler. Kepler was happy to move to Silesia because there the Protestants had not been under as much pressure as in other areas (although the territory was part of the Emperor's domain).

The religious battles eventually reached Sagan, where only a few among the nobility were Catholics. Although Wallenstein had taken a rather liberal attitude towards the confession he now felt obliged, for political reasons, to lead his subjects back into the Catholic religion. In November 1628 all citizens were ordered to become Catholic or leave. Kepler kept himself busy producing and printing annual calendars and almanacs, with weather forecasts and astronomical positions of the planets, which Wallenstein handed on to astrologers to learn from them about the future. Kepler was assisted in this task of producing these Ephemerides by Jakob Bartsch, who subsequently married his daughter Susanna on March 30, 1630, in Strasbourg [23]. Altogether, Ephemerides for the period 1621-1639 were produced and published; those for the years 1621-1629 included Kepler's day-to-day weather observations. Parts of these publications were dedicated in turn to the Upper Austrian representatives, to whom Kepler felt he owed a debt of gratitude, and to Wallenstein.

During the printing of the Ephemerides, yet another book Somnium seu Astronomia Lunari (Dream or Astronomy of the Moon) - ostensibly a book of science fiction - went to press. The book has a long history; it dates back to 1608 when he had compiled some thoughts on how various celestial motions would appear to an inhabitant of the Moon. The book is full of fantasy but it does contain a large body of appended notes that bear witness to the author's great intellectual ability and knowledge. Publishing was not completed until 1634.

In August 1630, amid many political intrigues, mistrust and jealousy and with war raging almost everywhere, Wallenstein was dismissed. Shortly afterwards, in October, Kepler set out for Linz to tray to obtain the interest on his investments that he was owed; following some correspondence, he was told to present himself on November 11 when there would be a chance of him getting satisfaction. En route he visited colleagues in Leipzig and Nuremberg, and on November 2 he arrived in Regensburg. A few days after his arrival he became ill although he attached little significance to it; he had often suffered bouts of fever. His condition worsened and he began to lose consciousness and became delirious. At noon on November 15 he died and two days later he was laid to rest in the Protestant cemetery of St. Peter. Numerous friends and acquaintances and illustrious members of society paid tribute.

Perhaps, in a manner somewhat typical of the man himself, the burial place was swept away within a year during the defense of Regensburg. A year later, more destruction of the churchyard occurred during further military actions. Whatever remained was demolished and ploughed up when, in 1634, the Bavarian and imperial troops reconquered the city. In the misery that followed the war no one was concerned about the fate of the dead and so, barely a few years after he died the resting place of his bones is no longer known. Tycho Brähe rests in the Tyn church in Prague, Galileo is buried in the church of Sante Croce in Florence and Newton lies among the British heroes in Westminster Abbey in London; yet Johannes Kepler, who was no less gifted than these mighty three, has no tombstone. It is as though fate required that he suffer in death much as he suffered during life.

Shortly before his death Kepler composed the following epitaph:

Did he have a premonition of his death?

During his lifetime, he had few followers. His contemporaries did not accept his theories of 'harmonic cosmology' nor his three laws. Even the great scientists Galileo and Descartes essentially ignored Kepler's contributions; in his Dialogues of 1632 Galileo said that the true shape of planetary orbits was unknown! Isaac Newton may be considered a worthy successor to Kepler. For, starting from Kepler's three laws together with the idea of gravitational attraction, Newton achieved the dream of Johannes Kepler in his Philosophiae Naturalis Principia Mathematica, i.e., the amalgamation of terrestrial and celestial physics.

FOOTNOTES

[1] Heinrich, who was two years younger than Johannes, was basically a ne'er-do-well. He was an epileptic and suffered brutal beatings. He was employed in a wide range of meager jobs in Austria, Belgium, Strasbourg, Mainz, Cologne and Prague, before returning home. He died at the age of forty-two years. His sister Margarete was gentle and was close to Johannes. She married a clergyman. His youngest brother Christoph, who was about fifteen years younger than Johannes, was an honorable man and he became a respectable craftsman - a tinsmith and pewterer - and citizen in Leonberg. He was also active in the militia, as a drill master for the Duke of Württemberg. The children, particularly Margarete and Christoph, stood by and helped their mother during her trial as a witch.

[2] At an early age he attempted poetry; later, he tried to write comedies and lyrical poems in ancient poetical form. He was especially partial to riddles and anagrams and he exercised his mind by memorizing long psalms.

[3] Michael Mästlin (1580-1635), an astronomer of some note, had tried unsuccessfully, like Brähe, to detect any parallax of the 1577 comet. Kepler and Mästlin continued to correspond frequently long after Kepler left Tübingen. Even after he had become famous, Kepler repeatedly expressed his gratitude and devotion publicly to his teacher.

[4] The Copernican system was disputed by both Roman Catholic and Protestant theologians. Theology in the 16th century had come to mean a literal acceptance of every word of the Scriptures; with the Protestants it was because they were unwilling to grant the Pope any authority, with the Roman Catholics it was because they wanted to define their doctrines within narrow boundaries. Thus, we find that Martin Luther (1483-1546) and other Protestant theologians of the time were highly critical of those who accepted Copernicus' theory. In Kepler's time, Copernicus' theory had not actually been outlawed, probably because Osiander's preface to De Revolutionibus referred to the theory as a hypothesis, to be used merely as a method of calculation. However, as time progressed, concern for the theory developed from a mathematical hypothesis, which concerned few people, to a question of the position of the Earth, i.e., whether it was the most important part of the Creation or a relatively insignificant part.

[5] Württemberg was using the (old) Julian calendar and so when he arrived in Austria, where the 'new' calendar was in use, he 'lost' ten days.

[6] It is unlikely that Kepler and Galileo knew each other prior to this correspondence. For example, Galileo had shown little interest in astronomy prior to 1597. Furthermore, the wording of a subsequent letter (dated September 1597) from Kepler to Michael Mästlin, after he had received Galileo's note, strongly suggests that the latter's name was new to him. Galileo's note is an important document in itself because he first declares his own support of the Copernican theory - see Galileo's biography in this series.

[7] In a letter to Michael Mästlin, Brähe was somewhat more critical of Kepler's ideas.

[8] To stop dissenting members of the Church becoming Protestants, the Catholic Church began a series of reform measures known as the Counter Reformation. The Counter Reformation was a direct response to the Protestant Reformation that had been declared in 1517 by Martin Luther over growing disenchantment with the Catholic Church.

[9] Protestant preachers still remained here and there in the castles. However, if an ordinary subject of a prince asked for and received the sacrament, the preacher was banished.

[10] Brähe and Kepler disagreed fundamentally on the theory of Copernicus. Brähe had proposed a new system, the Tychonic system, that was an amalgamation of Ptolemy and Copernicus. Furthermore, he accepted the Ptolemaic idea of the rotation of the heavens. Kepler rejected these ideas completely; he said he was ... "absolutely convinced" ... of the Copernican theory. However, he was worried about the lack of parallax of the stars as the Earth orbits the Sun; he was particularly about the large distance that the lack of observation implied. To try to get to the bottom of this difficulty, he started to make observations himself and requested similar ones from Mästlin, Brähe and Galileo.

[11] One promise Kepler received from Brähe was that he would make every effort to seek a decree from the Emperor requesting that Kepler return to Bohemia to help with the publication of his (i.e., Brähe's) work, provided the authorities in Graz agreed and continued to pay Kepler's salary. Brähe had negotiations with the Imperial Vice-Councilor who promised to ask the Emperor to talk with Archduke Ferdinand and Styrian representatives on Kepler's behalf. Apparently, the question where Kepler was to live was left open.

[12] Brähe now needed Kepler to join him since his long time collaborator, Longomontanus, had recently left for good to return to Denmark.

[13] During Kepler's first visit to Prague, Brähe had him work with Christen Sörensen Longberg (or Longomontanus) on a theory of the planet Mars. He wrote:

[14] It was fortunate that Mars had been selected for study because it has a relatively large eccentricity, i.e., deviation from a circular orbit. The eccentricity of Mars is 9.3%; the values for Venus, the Earth, Jupiter and Saturn are 0.7%, 1.7%, 4.8% and 5.6%, respectively.

[15] Kepler had even used some of Fabricius' measurements of the position of Mars during 1602 and 1604 to supplement those of Brähe.

[16] For example, in 1604 Kepler had agreed not to publish anything based on Brähe's data without Tengnagel's consent until the Rudolphine Tables were finished.

[17] For example, Rudolph, who succeeded Maximillian II in 1575, and had received his education in Spain, championed the Catholic side. In 1602 it was decreed that only Catholics and Ultraquists were to be tolerated. (In fact, all three of Kepler's children were baptized as Ultraquists.)

[18] Galileo found that Saturn does not appear as a simple disc. He thought he saw a main body with two small spheres near either side; he interpreted the latter as moons. It would be nearly half a century later that Christian Huygens would provide he correct interpretation, i.e., that Saturn has a ring system.

[19] Apparently, Reinbold recalled a time when she visited Frau Kepler and became ill after taking a drink. She spread the rumor that Frau Kepler was a witch who had tried to poison her with a magic drink. In fact, Reinbold's illness arose after an abortion which had been carried out to hide her promiscuity.

[20] Margareta Regina was born on January 7, 1615 and died on September 8, 1617 of a cough, consumption and epilepsy. Katherina was born of July 31, 1617 and died on February 9, 1618 of similar symptoms. Sebald was born on January 28, 1619 and died on June 15, 1623 from smallpox. The three surviving children were Cordula (born January 22, 1621), Fridmar (born January 24, 1623) and Hildebert (born on April 6, 1625).

[21] In March 1616 Copernicus' De Revolutionibus was placed on the Index of prohibited books by the Roman Inquisition. In the summer of 1619 Kepler received the news that the first part of his Epitome, which had appeared in 1617, had been placed also on the Index of prohibited books by the holy officials (on May 10, 1619). Kepler was concerned that censorship might extend into Austria so he would be unable to find a publisher for any of his material (and particularly his Harmonice. However, some of his correspondents in Italy told him that in Italy all books written by distinguished German scholars would be secretly bought and read more eagerly if banned!

[22] Today, we express the third law as:

[23] Apparently, Kepler was not present at the wedding since he was at home with his wife, who was about to give birth (to a daughter, Anna Maria, born on April 18).

REFERENCES

I have used material from the following sources (sometimes quoting verbatim):

Books

Max Caspar Kepler (Dover Publications, Inc., New York, 1993). It is a corrected and expanded edition of the work originally published in 1959 by Abelard-Schuman Ltd., London)

Patrick Moore Watchers of the Stars (Michael Joseph Ltd., 1974)

A. Pannekoek A History of Astronomy (Dover Publications, Inc., New York, 1989). It is an unaltered republication of the English translation, originally published in 1961 by George Allen & Unwin Ltd., London. The work was first published by Wereld-Bibliotheek in 1951 with the title De Groei van ons Wereldbeeld.